Esther Inglis-Arkell

People who live with pets notice that some animals moult in the spring and fall. Losing feathers or fur is unattractive, but it leads to a beautiful new coat in which to survive the winter or to attract a mate. It turns out humans also moult and grow attractive new coats of hair.

Hair grows in a three-phase process. During the anagen phase, the tiny bud of the follicle sinks down and grows a new robust follicle underneath the shriveled follicle of the last hair. The new hair grows up alongside the old one, peeping out of the same orifice, until the old, rootless hair finally falls out. The process of growth is short for body hair like eyebrows and eyelashes, but can last for years in scalp hair. The amount of years can be anywhere from two to six, which is why some people can't grow their hair past their shoulders, while others can grow it out long enough to sit on. After the anagen phase come the catagen, or "resting" phase. The hair stops growing, and beneath the skin the follicle begins to shrink, losing depth and width. Finally, there is the telogen phase, when the hair stops growing, and can come loose from the head entirely.

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After the first hair growth of infancy, human follicles don't sync up, but at any one time most of the hair is in the anagen phase. There are times, however, when the percentage of hair in anagen increases. A study, which admittedly used only Caucasian men, found that among active people who spent a great deal of time outdoors, well over 90% of their follicles were in anagen in the early spring, and by the autumn only about 80% were in anagen. The autumn also doubled the rate of shed hairs. A second study of women in New York found that women grow more hair in the spring and moult in the fall, just like men do. And it's not just scalp hair that undergoes these changes. Men's beards kicked their growth up in the spring. As for body hair, an analysis of thigh hair found that in May and November about 80% of follicles were sprouting, while in March and August only 60% were in anagen phase.

It's possible that fall body hair might be an attempt to keep us warm, even though our body hair is paltry when compared to other animals. What's more interesting is the spring growth of hair. Spring is when nature gets it on. Might our bodies be trying to grow impressive pelts to attract the best mates? At a time when most women shave and wax, the idea of bushy body hair attracting a mate is intriguing.